The focal spot is the area of the target upon which the electron beam impinges. The energy of the electrons in the electron beam is mostly converted into heat (approximately 99 percent which is why materials such as Tungsten are used due to their high melting-points) and dissipated uniformly across the focal spot and anode surface. The x-rays produced at the anode comprise of less than one percent of the energy of the electrons in the electron beam.
A large focal spot is therefore useful to protect the tungsten target as the heat accumulates and dissipates within the area of focal spot. However, a small focal spot is required to achieve a good radiographic image quality.
Thus the line focus principle helps resolve this issue by stating that an angulation of the anode surface will result in an apparent decrease in the focal spot size
The apparent focal spot (projected focal spot) size can be determined by the sine of the angle of the anode surface (apparent focal spot size = real focal spot size * sin anode angle). The angle varies as per tube design with a range value of 6 degrees to about 20 degrees.
Limitation of the principle
There are two important aspects to consider with regards to target angle:
- size of the apparent focal spot
- area covered by the x-ray beam
Firstly, for a given apparent focal spot size, the real area covered by the electron beam is larger for smaller target angles which, as stated above allows a greater area over which to dissipate the heat.
Secondly, for a smaller target angle, the area covered by the x-ray beam will be smaller so it is not possible to cover large areas at smaller FFDs, therefore it can be appreciated that choice of target angle is a compromise between tube loading, geometric unsharpness and desired area to be covered by the useful beam. For practical purpose, at 40" FFD the anode angle should be no smaller than 15 degrees. A decrease in angle below six degrees will result in anode heel effect.
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Physics and imaging technology: x-ray
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